The invention relates to a method for processing a received signal, as well as to an optical sensor.
A known optical sensor in the form of a light grid is provided with a predetermined number of transmitter-receiver pairs which define beam axes or paths. The transmitter-receiver pairs are activated cyclically and successively by an evaluation unit. Each transmitter emits transmitting light rays which are guided onto the associated receiver if the beam path is clear. If at least one beam path is interrupted, a shut-down command is generated by the evaluation unit of the light grid, e.g. for shutting down a machine for which the surrounding area is monitored with the light grid.
Individual received signals are present at the outputs of the individual receivers. The receivers and the associated transmitters are successively activated during one cycle. As a result, the evaluation unit records a chronological course of a combined received signal that is composed of the individual received signals coming from the different receivers.
A light grid of this type is described, for example, in German Patent DE 39 39 191 C2 and is provided with a two-channel evaluation unit, having a microcontroller in each evaluation channel. An object detection signal is generated from the received signal in these microcontrollers. The object detection signal indicates whether an object is located in the area to be monitored which is defined by the beam axes. The respective microcontrollers in each evaluation channel are assigned separated analog circuits for amplifying and pre-processing the received signal.
The analog circuit of one evaluation channel in particular comprises a two-channel amplifier and two comparators, installed downstream thereof, which are set to different circuit thresholds. The output signals from the comparators are assigned to a monostable flip-flop circuit. The signal pulses present at the flip-flop are fed to a counter. The received signals are pre-processed with this circuit to detect contamination of the transmitters or receivers.
Analog circuits of this type have the disadvantage of being sensitive to external interfering influences, for example EMC (electromagnetic compatibility) interfering influences and thermal drifts of the individual components.
Such interfering influences lead to distorted and falsified received signals and, in the final analysis, also to incorrect object detections which consequently lead to an undesirable reduction in the detection sensitivity of the light grid.